Kaolin particle film applications can increase photosynthesis and water use efficiency of 'Ruby red' grapefruit leaves

Effects of foliar sprays of a kaolin clay particle film (Surround WP) on leaf temperature (T-lf), net gas exchange, chlorophyll fluorescence and water relations of sun-exposed leaves on field-grown grapefruit trees (Citrus paradisi L.) were studied during Summer and Fall 2001. Trees were sprayed twice a week for 3 weeks with aqueous suspensions of kaolin (Surround) at 60 g(.)L(-1). Physiological effects of kaolin application were most prominent around midday on warm sunny days than in mornings, evenings or cloudy days. Kaolin sprays increased leaf whiteness (62%), reduced midday leaf temperature (T-lf; approximate to 3 degreesC and leaf to air vapor pressure differences (VPD; approximate to20%) compared to water-sprayed control leaves. Midday reductions in T-lf and VPD were accompanied by increased stomatal conductance (g,) and net CO2 assimilation rates (A(CO2)) of kaolin sprayed leaves, suggesting that g, might have limited A(CO2) in water-sprayed control leaves. Midday photoinhibition of photosynthesis was 30% lower in kaolin-sprayed leaves than in control leaves. Midday water use efficiency (WUE) of kaolin-sprayed leaves was 25% higher than that of control leaves. However, leaf transpiration and whole-tree water use were not affected by kaolin film sprays. Increased WUE was therefore, due to higher A(CO2). Leaf intercellular CO2 partial pressures (C-i) were similar in control and kaolin-sprayed leaves indicating that stomatal conductance was not the major cause of reduced A(CO2). These results demonstrate that kaolin sprays could potentially increase grapefruit leaf carbon uptake efficiency under high radiation and temperature stress.